About ZenersZener diodes are very common for basic voltage regulation tasks. They are used as discrete components, and also within ICs that require a reference voltage. Zener diodes (also sometimes called voltage reference diodes) act like a normal silicon diode in the forward direction, but are designed to break down at a specific voltage when subjected to a reverse voltage.

All diodes will do this, but usually at voltages that are unpredictable and much too high for normal voltage regulation tasks. There are two different effects that are used in Zener diodes ...

Impact ionisation (also called avalanche breakdown)

Zener breakdown

Below around 5.5 Volts, the zener effect is predominant, with avalanche breakdown the primary effect at higher voltages. While I have no intention to go into specific details, there is a great deal of information on the Net (See References) for those who want to know more. Because the two effects have opposite thermal characteristics, zener diodes at close to 5.5V usually have very stable performance with respect to temperature.

Using Zener DiodesFor reasons that I don't understand, there is almost no information on the Net on exactly how to use a zener diode. Contrary to what one might expect, there are limitations on the correct usage, and if these are not observed, the performance will be much worse than expected. Figure 1 shows the standard characteristics of a zener, but as with almost all such diagrams omits important information.

Figure 1 - Zener Diode Conduction

So, what's missing? The important part that is easily missed is that the slope of the breakdown section is not a straight line. Zeners have what is called 'dynamic resistance' (or impedance), and this is something that should be considered when designing a circuit using a zener diode.

The actual voltage where breakdown starts is called the knee of the curve, and at this region the voltage is quite unstable. It varies quite dramatically with current, so it is important that the zener is operated above the knee, where the slope is most linear.

Some data sheets will give the figure for dynamic resistance, and this is usually at around 0.25 of the maximum rated current. Dynamic resistance can be as low as a couple of ohms at that current, with zener voltages around 5 - 6V giving the best result. Note that this coincides with the best thermal performance as well.

This is all well and good, but what is dynamic resistance? It is simply the 'apparent' resistance that can be measured by changing the current. This is best explained with an example. Let's assume that the dynamic resistance is quoted as 10 ohms for a particular zener diode. If we vary the current by 10mA, the voltage across the zener will change by ...

V = R * I = 10Ω * 10mA = 0.1V (or 100mV)

So the voltage across the zener will change by 100mV for a 10mA change in current. While that may not seem like much with a 15V zener for example, it still represents a significant error. For this reason, it is common to feed zeners in regulator circuits from a constant current source, or via a resistor from the regulated output. This minimises the current variation and improves regulation.

Manufacturers' data sheets will often specify the dynamic resistance at both the knee and at a specified current. It is worth noting that while the dynamic resistance of a zener may be as low as 1.5 ohms at 25% of maximum current, it can be well over 500 ohms at the knee, just as the zener starts to break down. The actual figures vary with breakdown voltage, with high voltage zeners having very much higher dynamic resistance (at all parts of the breakdown curve) than low voltage units. Likewise, higher power parts will have a lower dynamic resistance than low power versions (but require more current to reach a stable operating point).

Finally, it is useful to look at how to determine the maximum current for a zener, and establish a rule of thumb for optimising the current for best performance. Zener data sheets usually give the maximum current for various voltages, but it can be worked out very easily ...

For example, a 27V 2W zener can carry a maximum continuous current of ...

I = 2 / 27 = 0.074A = 74mA (less a small 'fudge factor', and at 25°C)

As noted in the 'transistor assisted zener' app note (AN-007), for optimum zener operation, it is best to keep the current to a maximum of 0.7 of the claimed maximum, so a 27V/2W zener should not be run at more than about 47mA. The ideal is probably about 25% of the maximum (17mA close enough, although reference 3 says 18.5mA), as this minimises wasted energy and ensures that the zener is operating within the most linear part of the curve. If you look at the zener data table below, you will see that the test current is typically between 25% and 36% of the maximum continuous current. The wise reader will figure out that this range has been chosen to show the diode in the best possible light, and is therefore the recommended operating current .

While none of this is complex, it does show that there is a bit more to the (not so) humble zener diode than beginners (and many professionals as well) tend to realise. Only by understanding the component you are using can you get the best performance from it. This does not only apply to zeners of course - most (so called) simple components have characteristics of which most people are unaware.

Remember that a zener is much the same as a normal diode, except that it has a defined breakdown voltage that is far lower than any standard rectifier diode. Zeners are always connected with reverse polarity compared to a rectifier diode, so the cathode (the terminal with the band on the case) connects to the most positive point in the circuit.

Zener ClampsOften, it is necessary to apply a clamp to prevent an AC voltage from exceeding a specified value. Figure 2 shows the two ways you may attempt this. The first is obviously wrong - while it will work as a clamp, the peak output voltage (across the Zeners) will only be 0.65V. Zeners act like normal diodes below their breakdown voltage, so the first figure is identical to a pair of conventional diodes.

Figure 2 - Zener Diode AC Clamp

In the first case, both zener diodes will conduct as conventional diodes, because the zener voltage can never be reached. In the second case, the actual clamped voltage will be 0.65V higher than the zener voltage because of the series diode. 12V zeners will therefore clamp at around 12.65V - R1 is designed to limit the current to a safe value for the zeners, as described above.

The important thing to remember is that zener diodes are identical to standard diodes below their zener voltage - in fact, conventional diodes can be used as zeners. The actual breakdown voltage is usually much higher than is normally useful, and each diode (even from the same manufacturing run) will have a different breakdown voltage.

Zener Diode Data
The following data is a useful quick reference for standard 1W zeners. The basic information is from the Semtech Electronics data sheet for the 1N47xx series of zeners. Note that a 'A' suffix (e.g. 1N4747A) means the tolerance is 5%, and standard tolerance is usually 10%. Zener voltage is measured under thermal equilibrium and DC test conditions, at the test current shown (Izt).

Type

VZ (Nom)

Izt

Rzt

Rz at ...

Current(mA)

LeakageuA

LeakageVoltage

PeakCurrent (mA)

Cont.Current (mA)

1N4728

3.3

76

10

400

1

150

1

1375

275

1N4729

3.6

69

10

400

1

100

1

1260

252

1N4730

3.9

64

9.0

400

1

100

1

1190

234

1N4731

4.3

58

9.0

400

1

50

1

1070

217

1N4732

4.7

53

8.0

500

1

10

1

970

193

1N4733

5.1

49

7.0

550

1

10

1

890

178

1N4734

5.6

45

5.0

600

1

10

2

810

162

1N4735

6.2

41

2.0

700

1

10

3

730

146

1N4736

6.8

37

3.5

700

1

10

4

660

133

1N4737

7.5

34

4.0

700

0.5

10

5

605

121

1N4738

8.2

31

4.5

700

0.5

10

6

550

110

1N4739

9.1

28

5.0

700

0.5

10

7

500

100

1N4740

10

25

7.0

700

0.25

10

7.6

454

91

1N4741

11

23

8.0

700

0.25

5

8.4

414

83

1N4742

12

21

9

700

0.25

5

9.1

380

76

1N4743

13

19

10

700

0.25

5

9.9

344

69

1N4744

15

17

14

700

0.25

5

11.4

304

61

1N4745

16

15.5

16

700

0.25

5

12.2

285

57

1N4746

18

14

20

750

0.25

5

13.7

250

50

1N4747

20

12.5

22

750

0.25

5

15.2

225

45

1N4748

22

11.5

23

750

0.25

5

16.7

205

41

1N4749

24

10.5

25

750

0.25

5

18.2

190

38

1N4750

27

9.5

35

750

0.25

5

20.6

170

34

1N4751

30

8.5

40

1000

0.25

5

22.8

150

30

1N4752

33

7.5

45

1000

0.25

5

25.1

135

27

1N4753

36

7.0

50

1000

0.25

5

27.4

125

25

1N4754

39

6.5

60

1000

0.25

5

29.7

115

23

1N4755

43

6.0

70

1500

0.25

5

32.7

110

22

1N4756

47

5.5

80

1500

0.25

5

35.8

95

19

1N4757

51

5.0

95

1500

0.25

5

38.8

90

18

1N4758

56

4.5

110

2000

0.25

5

42.6

80

16

1N4759

62

4.0

125

2000

0.25

5

47.1

70

14

1N4760

68

3.7

150

2000

0.25

5

51.7

65

13

1N4761

75

3.3

175

2000

0.25

5

56.0

60

12

1N4762

82

3.0

200

3000

0.25

5

62.2

55

11

1N4763

91

2.8

250

3000

0.25

5

69.2

50

10

1N4764

100

2.5

350

3000

0.25

5

76.0

45

9

Table 1 - Zener Characteristics, 1N4728-1N4764

Izt = zener test current

Rzt = dynamic resistance at the stated test current

Rz = dynamic resistance at the current shown in the next column

Leakage current = current through the zener below the knee of the zener conduction curve, at the voltage shown in the next column

Peak current = maximum non-repetitive short term current (typically < 1ms)

Continuous current = maximum continuous current, assuming that the leads at 10mm from the body are at ambient temperature

Figure 3 - Zener Diode Temperature Derating

Like all semiconductors, zeners must be derated if their temperature is above 25°C. The above graph shows the typical derating curve for zener diodes, and this must be observed for reliability. Like any other semiconductor, if a zener is too hot to touch it is hotter than it should be. Reduce the current, or use the boosted zener arrangement described in AN-007.